1. Technical Field
The disclosure relates in general to a thermoelectric apparatus and a method of fabricating the same, and more particularly to a thermoelectric apparatus capable of increasing power output of thermoelectric modulec and a method of fabricating the same.
2. Description of the Related Art
The problem of energy shortage has made the development of renewable energy technologies become an important issue. Take the vehicles for example. The waste heat of the engine of some vehicles amounts to about 33% of the power, and fuel consumption can be reduced if the exhaust heat can be used for power generation by way of thermoelectric temperature difference. Additionally, factories and households both discharge a large amount of waste heat. Therefore, how to recycle the discharged waste heat has also become an important issue. Currently, there still lacks suitable technologies for recycling the waste heat, particularly the waste heat generated by low temperature thermal source, and the energy loss is considerable.
Thermoelectric device is a device for converting thermal energy to electrical energy and vice versa. Based on the properties of thermoelectric conversion, two fields of application, namely heating/cooling capacity and power generation, are created. According to the Seebeck effect, when an n-type semiconductor material and a p-type semiconductor material that are electrically coupled contact different temperatures at the p material connection end and the n material connection end respectively, energy is transferred, and electrical current is generated in the thermocouple. The thermoelectric conversion can be used in waste heat power generation for generating power according to the temperature difference at the two ends of a thermoelectric device which is a heat generator. On the other hand, according to the Peltier effect, when a direct current is applied to a thermoelectric device, heat absorption and heat dissipation will occur to the two ends of the thermoelectric device respectively, and such principle can be used in the cooling or heating technologies.
Referring to FIG. 1, a cross-sectional view of an apparatus using a conventional thermoelectric device is shown. In general, a conventional thermoelectric conversion apparatus is composed of a p-type thermoelectric material 101, an n-type thermoelectric material 102, conductive metal layers 111a and 111b, a top substrate 121a and a bottom substrate 121b. The p-type thermoelectric material 101 and the n-type thermoelectric material 102 are lump-shaped, and both of the top substrate 121a and the bottom substrate 121b possess electrical insulation and high thermal conductivity. The functions of the thermoelectric device are mainly determined by the properties of the thermoelectric materials 101 and 102. As indicated in FIG. 1, the p-type thermoelectric material 101 and the n-type thermoelectric material 102 are normally vertically type, and are connected in series via the conductive metal layers 111a and 111b. The top and bottom substrates 121a and 121b with electrical insulation and high thermal conductivity, for example, are made of ceramic material. In the application of thermoelectric cooling module, the inputted direct current flows in the p-type thermoelectric materials 101 and the n-type thermoelectric materials 102 in a direction (vertically flow) parallel to that of thermal flow (vertically transferring) of the conversion device, and the thermoelectric cooling module generates temperature difference, and absorb and dissipate the heat at the top and the bottom, respectively. Take power generation by way of temperature difference for example. The directions of the thermoelectric device temperature difference and thermal flow are still parallel to the flow direction of the current generated in the thermoelectric materials. However, the cooling capacity and power generation efficiency of the thermoelectric devices with conventional structure are subjected to the restriction in figure of merit (ZT) of the lump-shaped thermoelectric materials, and the maximum cooling capacity of some commercial thermoelectric devices reach 3˜5 W/cm2 only, and the power generation efficiency of such thermoelectric device is about 2˜3% when the temperature difference between the cooling end and the heating end is 200. To increase the power generation efficiency of the thermoelectric device, thermoelectric materials with high ZT value has been proposed for using in the thermoelectric device.
Despite many studies have been proposed for enhancing the properties of thermoelectric materials and improving the efficiency of thermoelectric devices, the achievements are still limited. When the ZT value of a thermoelectric material is smaller than 1, the performance of the thermoelectric device will be restricted. In 1993, Professors Hicks and Dresselhaus et al of the MassaChusetts Institute of Technology of USA suggested that the ZT value could be effectively increased if the scale of thermoelectric materials is downsized to nano level. In 2001, Venkatasubramanian et al of the RTI (Research Triangle Institute) of USA disclosed that the ZT value of the p-type Bi2Te3/Sb2Te3 super-lattice thin film can reach 2.4 at room temperature, marking a breakthrough for the bottleneck that the ZT is about 1.
In the field of material technology, the thermoelectric materials with high thermoelectric performance (the ZT value) continue to be developed. It is also an important direction in the development of technology to design a thermoelectric devices or structures capable of generating larger volume of power.